The following relates to the nuclear reactor fuel handling arts, nuclear fuel rod loading arts, nuclear fuel pellet handling arts, and related arts.
The nuclear reactor core of a nuclear reactor is typically assembled as a set of fuel rods. Each fuel rod comprises a hollow cylindrical cladding filled with fuel pellets (and/or possibly pellets of other types, such as spacer pellets, and/or pellets containing a neutron poison) and capped by end plugs. The composition, enrichment, and arrangement of the pellets loaded into a given fuel rod are chosen to provide designed reactivity or other characteristics. For example, one illustrative reactor core design includes fuel rods comprising uranium dioxide (UO2) of various enrichment levels, and spike fuel rods comprising UO2Gd2O3 for reactivity control. The enrichment (i.e. fissile 235U concentration or fraction) of fuel pellets may be varied over the length of the fuel rod and/or among different fuel rod types. Typically, the pellets loaded into a given fuel rod are arranged as discrete axial zones along the length of the fuel rod, with each zone having a defined number of fuel pellets of the same defined composition and 235U enrichment. The loading of the fuel pellets into the fuel rods must be done with exacting precision so that the assembled nuclear reactor core has the designed characteristics. Regulatory rules and best practices dictate that the fuel loading be documented and traceable so that the composition of each fuel rod is known and traceable throughout the manufacturing, shipping, and reactor fueling processes.
In so-called vibratory pellet loading methods, multiple pieces of empty fuel rods (with welded lower end plug) are placed on a large vibratory table. The upper ends of the rods fit within a transition element. The transition element guides the pellets from a v-trough into the rods. Pellets are arranged in the v-trough in the correct length in front of each transition. The table is then excited (i.e. vibrated) such that the pellets move under their own inertia into the rods. The process is repeated until all the segments are loaded.
The handling of large pellet trays is a significant challenge for operators. The trays must be loaded and unloaded for each enrichment zone. The loading time increases with the number of zones. Multiple rods are loaded at the station to minimize cycle time. Pellets of different types for the different axial zones may be arranged in the v-trough, raising the possibility of inadvertent mixing of pellets of different types.
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